This disclosure relates to optical sweeping systems, and particularly to non-impact printers and facsimile machines of the laser type.
In optical sweeping systems, a collimated light beam, for example, a laser light beam, is reflected by a rotating, multifaceted mirror. The rotating mirror causes the reflected light beam to sweep periodically in fan-like fashion across a moving target surface, the end of the beam tracing out a plurality of straight lines thereon. By sweeping, it is meant that the direction of the light beam reflected from a side of the mirror changes over a fixed angle as the mirror rotates until the light beam moves off the side of the mirror onto the next side of the mirror to begin the sweep all over again. The direction of the beam changes at a constant rate, that is, the direction of the beam changes by the same angular amount for a given time interval at either end of the sweep, at the center of the sweep or anywhere else during the sweep. Because of this and because the distance to the straight line path on the target surface from the reflection point on the mirror is greater at the ends of the sweep than at the middle, the end of the reflected beam covers a greater distance along the straight line path at the ends than at the middle of the path during the same time interval. This is often referred to in the art as the tangential velocity of the beam and it varies during the sweep being greater at the ends of the sweep and less in the middle.
In laser printers, the light beam is modulated before reflection in accordance with selected patterns of bit signals which represent alphanumeric characters which are stored in a character generator memory as a matrix of ones and zeros. A character clock signal gates the individual bit signals from the character generator and the bit signals are transmitted to an RF signal source which, for example, transmits RF signals when high bit signals (ones) are received and no RF signals when low bit signals (zeros) are received. Each sweep of the light beam is modulated in accordance with at least one row of ones and zeros of a plurality of matrices stored in a character generator memory for imaging as a portion of a line of alphanumeric characters on a photosensitive surface.
The RF signals are transmitted to a light beam modulator which is positioned in the path of the collimated light beam and which causes a portion of the light beam to be diffracted through a specific angle (called the Bragg angle) along a deflected path when RF signals are present at the modulator. The portion of the beam traveling along the deflected path is called the first order beam while the undeflected beam is called the zero order beam. The zero order beam is always present although with less energy when the first order beam is present. Together, the first and zero order beams form a modulated light beam.
The modulated light beam then passes through an optical system that controls the focus and size of the beam, and directs the beam to a rotating multifaceted mirror where the beam is swept as described above. As the modulated light beam follows the straight line path on the photosensitive surface during a sweep, the zero order beam is prevented from impinging on the photosensitive surface. When it is desired to image a dot along the straight line path, the first order beam is switched on in a manner as described above. Otherwise, a space is left on the straight line path. If the character clock signal which gates the individual bit signals from the character generator which causes the switching on of the first order beam, has a constant frequency, then the separation between adjacent dots and spaces at the ends of the straight line path is greater than at the center of the straight line path because of the variation in tangential velocity of the end of the sweeping light beam which was described above. This causes spreading of the subsequently imaged characters located at the ends of parallel straight line paths on the photosensitive surface. That is, characters imaged at the ends of the photosensitive surface are wider than the same characters imaged at the center. This results in nonuniform printing which gives an undesirable appearance and result.
U.S. Pat. No. 3,835,249 (Dattilo et al) discloses a synchronization device for generating a real time synchronization signal for utilization with a scanning light beam. The device is summarized at columns 1 and 2 of the patent. It includes: means for splitting the main scanning beam; an optical grating; an optical system having first and second optical foci; and a light detection device. Light split from the main scanning beam passes through the optical grating before impinging on the light detection device located at the second foci of the optical system. The output signal from the light detection device may be utilized to clock information into the light beam by modulating it or to clock information from the light beam imparted thereto by scanning a document.
The periodic spacing of optical grating lines along a straight line provides information with respect to the tangential velocity as it varies along the straight line path. In the synchronization device disclosed in U.S. Pat. No. 3,835,249, the output of the light detection device after amplification, limiting and clipping, is used either directly or after frequency doubling as a clocking signal. If the output of the light sensitive device is used directly as a clocking signal, then the number of optical grating lines must be the same as the number of dot spaces in a line of sweep. Such a grating would be difficult to fabricate for existing sweeping systems employing greater than 200 dot spaces per inch density while still minimizing the size of the synchronization system. This would be true even if the spacing between optical grating lines were doubled. Presumably, the frequency doubling of the output could be cascaded any number of times to thereby minimize the number of grating lines. However, in such a system the resultant character generator clocking signal rate would be an integer multiple of two times the number of grating lines occurring in the optical grating. This in turn requires that the fonts used for storing alphanumeric characters be related to the optical line grating since the grating determines the clocking rate. It is desirable that flexibility be maintained in the selection of character fonts to enable use of the optical sweeping device for a maximum number of applications. It is undesirable, therefore, to restrict the font description that can be used with a given optical sweeping device by the selection of a particular spacing in an optical grating as required by U.S. Pat. No. 3,835,249.
In addition to U.S. Pat. No. 3,835,249 described above, U.S. Pat. No. 4,019,186 (Dressen et al) relates to scanning beams in non-mechanical printers. U.S. Pat. No. 4,019,186 discloses a light beam motion pick up device comprising a light transmission rod having a plurality of marks thereon. The device provides timing pulses to aid in the printing of characters on a recording surface at uniform intervals along a line. A portion of the scanning light beam is scanned along the rod and whenever it strikes one of the plurality marks it is scattered and the scattered light travels inside the rod to a photo-electric element which provides timing signals after amplification. However, in order to provide a clocking signal for each dot or space forming a character in a dot matrix printer it would be necessary to provide a mark for each such dot or space along the length of the light transmission rod. This could amount to as many as 200 or more marks per inch.